The present invention relates to an electrophotographic photoreceptor composed of a charge transport layer, a charge generation layer and a surface protection layer sequentially laminated on a conductive substrate wherein the charge transport layer comprises a resin binder having a molecular weight at less than 20,000 and not more than 45,000.
As a photoconductive material for an electrophotographic photoreceptor (hereinunder referred to as "photoreceptor"), an inorganic photoconductive material such as selenium and a selenium alloy, an inorganic photoconductive material such as zinc oxide and cadmium sulfide dispersed in a resin binder, an organic photoconductive material such as poly-N-vinyl carbazole and polyvinyl anthracene, an organic photoconductive material such as a phthalocyanine compound and a bisazo compound dispersed or vacuum deposited in a resin binder has conventionally been used.
A photoreceptor is required to have the ability to accept the surface charges while in a dark environment, the ability to generate charges when exposed to light and the ability to transport the generated charges. A single-layer type photoreceptor has a single layer which has these functions together. A laminated photoreceptor has a lamination of layers having separated functions, namely, a layer mainly contributing to generation of charges and a layer contributing to accepting of the surface charges in the dark place and transportation of charges at the time of receiving light.
In order to form an image by electrophotography using these photoreceptors, a Carlson system copier, for example, may be used. In this system, an image is formed by charging a photoreceptor by corona discharge in the dark, and exposing the surface of the charged photoreceptor to light so as to form an electrostatic latent image of the letters, pictures, etc. of the material to be copied, developing the electrostatic latent image with toner, and fixing the developed toner image on a base such as paper. In the photoreceptor after transferring the toner image, static charges are eliminated, and the remaining toner is removed and discharged by light for the reuse of the photoreceptor.
Electrophotographic photoreceptors using an organic material have recently been put to practical use due to the merits of plasticity, thermal stability, film forming properties and the like. For example, disclosed are photoreceptors composed of poly-N-vinyl carbazole and 2,4,7-trinitrofluorene-9-one (U.S. Pat. No. 3,484,237), photoreceptors containing an organic pigment as the main ingredient (Japanese Patent Laid-Open No. 37543/1972) and photoreceptors containing a eutectic complex consisting of a dye and a resin (Japanese Patent Laid-Open No. 10735/1972).
Organic materials have many merits which inorganic materials do not have, but all the properties electrophotographic photoreceptors are required to have are not satisfied in the present state of art. Particularly, a positive charging type photoreceptor composed of a conductive substrate, a charge transport layer, a charge generation layer and a surface protection layer laminated in that order has problems in charging capacity and charge holding capacity. More specifically, since holes run more easily than electrons in many charge transport layers, the charging capacity and the charge holding capacity of a positive charging type photoreceptor are lower than those of a negative charging type photoreceptor.
It is an object of the present invention to eliminate the above-described problems in the prior art and to provide a positive charging type electrophotographic photoreceptor having excellent charging capacity and charge holding capacity.